Notify trakt user stopped watching video on scrobble failure.

When a scrobble fails, check if the failure was due to the movie/episode already being scrobbled.  If it was, send a cancel watching call to clear out the currently watching.
Add returnOnFailure flag to traktRequest to support this.
Merge pull request #86 from nate1280/scrobbling_fix

Notify trakt, user stopped watching video on scrobble failure.